Hydroformylation, earlier termed the oxo reaction, is an established process used by the chemical industry for converting alkenes to aldehydes, and sometimes alcohols, by reaction with hydrogen and carbon monoxide. Typically a feed stream constituted of alkenes, hydrogen and carbon monoxide is reacted over soluble rhodium- or cobalt-based transition metal catalysts at relatively low temperatures and pressures. Rhodium catalysts, the catalysts of choice for hydroformylation reactions, have long been recognized as more active than cobalt for promoting the "oxo" reaction, especially at low temperatures and pressures, even when the catalyst is used in relatively low concentrations. Rhodium catalysts, which produce primarily aldehydes, have also provided generally good selectivities in the production of linear aldehydes. Rhodium catalysts, however, require, inter alia, the use of an excess of a phosphine ligand in the reaction to stabilize the catalyst against the formation of decomposition products, and to maintain acceptably high product selectivities.
In U.S. Pat. No. 3,939,188 to Gary B. McVicker there is disclosed a laundry list of complex zero-valent rhodium catalysts useful for conducting hydroformylation reactions. These catalysts are represented by the formulae [(L)(L')Rh.sup.0 ].sub.2, [(L.sub.3)Rh.sup.0 ].sub.2, [L'.sub.2 Rh.sup.0 ].sub.2, [(L")Rh.sup.0 ].sub.2, where L is a monodentate ligand, L' is a bidentate ligand, and L" is a tri- or quadradentate ligand, wherein L, L', and L" can be the same or different and each one is selected from the group consisting of:
1. R.sub.3 Q, R.sub.2 R'Q, (RR'R")Q PA1 2. R.sub.2 Q(CR.sub.2 ').sub.x Q'R.sub.2 PA1 3. R.sub.2 Q(CR.sub.2 ')Q'(R")(CR.sub.2 ').sub.x QR.sub.2 PA1 4. R.sub.2 Q'(CR.sub.2 ').sub.x Q(R")(CR.sub.2 ').sub.x Q(R")(CR.sub.2 ').sub.x Q'R.sub.2 PA1 5. RQ'[(CR.sub.2 ').sub.x QR.sub.2 ].sub.2 PA1 6. Q'[(CR.sub.2 ').sub.x QR.sub.2 ].sub.3 PA1 X=Cl, Br, I, F; preferably Cl, Br, or I
wherein R, R', and R" can be the same or different and each is selected from the group consisting of hydrogen, F, Cl, Br, and I, C.sub.1 to C.sub.20 alkyl, C.sub.1 to C.sub.20 alkoxy, C.sub.3 to C.sub.8 cycloalkyl, C.sub.3 to C.sub.8 cycloalkoxy, phenyl, phenyl substituted with F, Cl, Br, and I, phenyl substituted with C.sub.1 to C.sub.20 alkyl, phenyl substituted with C.sub.3 to C.sub.8 cycloalkyl, phenyl substituted with C.sub.1 to C.sub.20 alkoxy, oxyphenyl, oxyphenyl substituted with C.sub.3 to C.sub.8 cycloalkyl, oxyphenyl substituted with F, Cl, Br, and I, oxyphenyl substituted with C.sub.3 to C.sub.8 cycloalkoxy; Q and Q' can be the same or different and each is selected from the group consisting of P, As, Sb, and x is an integer ranging from 1 to 5.
These catalysts were prepared in accordance with one or the other of the following reactions: EQU 2(L).sub.3 Rh.sup.I X+excess Mg(Hg).fwdarw.[(L).sub.3 Rh.sup.0 ].sub.2 +MgX.sub.2 EQU 2(L)(L')Rh.sup.I X+excess Mg(Hg).fwdarw.[(L)(L')Rh.sup.0 ].sub.2 +MgX.sub.2 EQU 2(L').sub.2 Rh.sup.I X+excess Mg(Hg).fwdarw.[(L').sub.2 Rh.sup.0 ].sub.2 +MgX.sub.2 EQU 2(L")Rh.sup.I X+excess Mg(Hg).fwdarw.[(L")Rh.sup.0 ].sub.2 +MgX.sub.2
The reduction of the rhodium(I) complex is performed in an inert atmosphere, at low to moderate temperature and pressure, with an excess of magnesium amalgam in the presence of solvent.
These catalysts, like other homogeneous catalyst systems employed for hydroformylation reactions, require the use of an excess of phosphine ligand to stabilize the catalyst, suppress decomposition reactions, and maintain acceptable selectivities. Better stability, particularly better stability without the use of an excess of a phosphine ligand, and improved selectivity in obtaining higher yields of the desired products are desired by the industry. Nonetheless, despite these shortcomings the Rh/PPh.sub.3 catalyst system continues in wide use throughout the chemical industry.